Norwegian research scientists are now working on the concept of storing electricity at the bottom of the sea. The energy will be stored with the help of high water pressure. It’s a new idea invented by a German engineer who has spent much of his professional life working in aerospace technology.
Rainer Schramm, inventor and founder of the company Subhydro AS said, “Imagine opening a hatch in a submarine under water. The water will flow into the submarine with enormous force. It is precisely this energy potential we want to utilize. Many people have launched the idea of storing energy by exploiting the pressure at the seabed, but we are the first in the world to apply a specific patent-pending technology to make this possible.”
Schramm has joined forces with SINTEF, the largest independent research organization in Scandinavia to research the concept. “SINTEF has experts in the fields of energy generation, materials technology and not least offshore and deep-water technology, which means we have all the expertise we need in one place,” he said.
To use the water pressure at the seabed in practice, the mechanical energy is converted by a reversible pump turbine, as in a normal pumped storage hydroelectric plant.
Schramm explains, “A pumped storage power plant is a hydroelectric plant that can be “charged” up again by pumping the water back to the upper reservoir once it has passed through a turbine. This type of power plant is used as a “battery”, when connected to the power grid.”
In this pumped storage power plant a turbine will be connected to a tank on the seabed at a depth of 400-800 meters. The turbine is fitted with a valve, and when this is opened, water flows in and starts turning the turbine. The turbine drives a generator to produce electricity. One can connect as many tanks as one might wish. In other words, it is the number of water tanks that decides how long the plant can generate electricity, before the energy storage capacity is exhausted.
“When the water tanks are full, the water must be removed from the tanks,” Schramm explains. This is achieved by running the turbine in reverse, so that it functions as a pump. The process consumes energy from the power grid, just as when one charges an ordinary battery. Although a bit more energy is used to empty the water tanks than can be recovered from flooding them, the degree of efficiency of this type of power plant is just as high as that of a conventional, onshore plant. According to Schramm, calculations indicate an electric storage efficiency of approximately 80% per power emptying cycle.
Another advantage of the system is that equipment can be scaled according to users’ requirements, both as regards the turbine size and the number of water tanks. A plant of normal size will produce roughly 300 megawatts for a period of 7-8 hours. This is enough energy to supply just over 200,000 (British measure) households with electricity for the same time.
Schramm said, “We envisage that this type of storage plant will function well in conjunction with, for example, wind farms. At strong wind conditions, excess electricity is sent subsea to pump water out of the storage tanks. In periods with little wind, energy can be obtained from this underwater plant instead. The same applies to solar generation: the pumped storage power station can contribute to constant electricity production at night time when there is no sunshine to run a solar power plant.”
In addition to the number of tanks, the sea depth also determines the effectiveness of the plant: the deeper the equipment is located, the greater is the pressure difference between the sea surface and the seabed, and the more energy is stored in a single tank.
Schramm explained, “This is part of the reason why we want to try out the technology in Norway.” In his native country Germany the sea is too shallow for the system to be profitable, but there are many parts of the world where great water depths are located close inshore, such as the marine areas around Italy, Portugal and Spain, as well as North and South America.
This where SINTEF comes in. One of the challenges is to develop a type of concrete that can be used to cast the water tanks, which are placed on the seabed. Tor Arne Martius-Hammer at SINTEF Building and Infrastructure is an expert on strong, light concrete types.
Martius-Hammer explains the SINTEF work with, “The challenge is to find the optimal balance between strength and cost. If we achieve the goal of creating a concrete that will withstand at least 5 times as high loading as ordinary concrete, we can reduce the wall thickness by 75%. This is a critical factor. We need to reach production and installation costs which make storage of energy economical in relation to the price of electrical energy. One of the solutions SINTEF will work on is reinforcing the concrete with thin steel fibers instead of the normal steel rebar. This will result in a significant simplification of the production process. Concrete is in existence at present which can be used, but our job is to develop a cheaper alternative.”
It all seems elegantly simple to use gravity and pressure to achieve high energy storage efficiency. As the team in Scandinavia is figuring out, its much more of an engineering exercise of the extreme.
Eighty percent efficiency look quite attractive. No battery, chemistry problems, or life cycle issues other than wear and tear. The concrete tanks could last indefinably. The physics are quite simple with no problems such as compressing air that would loss energy to thermal loses. One simply needs to be near a deep body of water.
Lets hope the thin steel fibers replacing the normal steel rebar work out great.